I have installed JSPWiki in windows and deployed war file (extracted all contents) in Tomcat 4.1.3 and it works fine.
When i installed JSPWiki in SunSolaris and deployed extracted war contents under webapps directory.

UnsupportedClassVersionError means the VM does not recognize the class file format (which means, most of the time, that the JDK you use to run the software is older than the JDK that compiled the software).

Indeed, see the System Requirements in JspWikiDownload: JspWiki v2.2 and above require JDK1.4 or above. You probably have one of those on the Windows machine...